There is little direct evidence to help determine the concentrations of ambient inhalable particles that are associated with adverse respiratory health effects. The U.S. Environmental Protection Agency in 1987 promulgated standards for ambient inhalable particles, specifically those particles smaller than 10 micrometers in diameter (PM1O), in the absence of epidemiologic data supporting the levels chosen. This study aims to provide data in support of a PM,o standard by evaluating the respiratory effects of ambient inhalable particles in a subgroup of the population that is particularly susceptible to effects of air pollution, namely, children with asthma. Four hypotheses will be tested: 1) Ambient inhaled particles are associated with increased daily reporting of symptoms in asthmatic children. 2) Ambient inhaled particles are associated with daily changes in the lung function of childhood asthmatics. 3) The number of particles rather than the mass concentration of particles is a better predictor of acute symptoms or of acute worsening of lung function in childhood asthmatics. 4) Higher concentrations of indoor aeroallergens or indoor particles are associated with more frequent symptoms and increased likelihood of worsened lung function in childhood asthmatics. The initial two-year study nearing completion tested all elementary school children in the Vancouver Island community of Port Alberni and identified, by questionnaire and lung function testing, those having a physician's diagnosis of asthma, those with exercise induced bronchospasm and those with airways obstruction. The current study design involves following these approximately 250 children for one year with daily entries in a symptom diary and daily peak expiratory flow measurements. A time-series analysis will evaluate the association of daily PM10 and other daily measures of inhalable particles with daily symptoms and daily lung function. The findings of this study will be directly relevant to the support or modification of the current 24-hour standard for ambient inhalable particle concentration.